Modern gas turbines can operate with lean premix flames to meet emission specifications. To assure a stable combustion for all operating conditions, complex operating methods have been designed as for example known from EP0718470.
In addition to the low emissions, the market specification with regard to transient operation capabilities of gas turbines in utility use are becoming more and more stringent. Besides normal frequency support features, gas turbines in local electrical grids can be called for to maintain the grid frequency under various conditions, including emergencies, such as, loss of national grid connection or trip of a large consumer. Gas turbines should be capable of changing their power outputs very quickly in order to keep the generated power and the consumed power balanced in the local grid.
One prerequisite for fulfilling these transient operation specification involves combustion stability during rapid transients. However, lean premixing combustion, which is a known technology for dry NOx emission reduction, can have a relatively narrow flammability range. The flammability limits of a lean premix flame are for example much narrower than those of a diffusion flame; can be approximately one order of magnitude smaller. The flammability limits describe the stability of flames. In addition, water and steam injections are popular methods for power augmentation and for wet NOx reduction. They also affect combustion stability.
The flammability limits can easily be exceeded during fast transient operation if known operating methods are carried out based on measured values and with direct command to actuators and control valves.
FIG. 1 shows an example of a gas turbine control system in accordance with a known implementation. FIG. 1 shows an example of a conventional gas turbine control system 30, actuators 41-49 supply channels 21, . . . , 29 to a combustor 3. The control system itself includes a controller 10, and control lines 11-19. Based on measured operating conditions, and operating targets the controller 10 determines the command mass flows for i fuel flows {dot over (m)}fuel,iCMD, for j water/steam mass flow {dot over (m)}w/s,jCMD, and for k air mass flows {dot over (m)}air,kCMD. Based on these command mass flows the actuators for fuel supply 41, . . . , 43, the actuators for water/steam supply 44, . . . , 47, and the actuators for air supply change their position leading to a fuel gas mass flow at the actuators {dot over (m)}fuel,iACTUR, water/steam mass flow j at the actuators {dot over (m)}w/s,jACTUR, and air mass flow k at the actuator, i.e. VIGV {dot over (m)}air,kACTUR.
Ideally the mass flows entering the combustor 3 follow the commanded mass flow strictly. However this is not the case in a real engine because the fuel, water/steam and air supply channels have different shapes and volumes, and hence, possess different system dynamics.
Due to the different dynamics of the actuators, and of the fuel supply channels 21, . . . , 23, the water/steam supply channels 24, . . . , 26, and the air supply channels 27, . . . , 29, changes in the actual i fuel mass flows {dot over (m)}fuel,iCMBST, the actual j water/steam mass flows reaching the combustor inlet {dot over (m)}w/s,jCMBST, and the actual k air mass flows {dot over (m)}air,kCMBST are not synchronized, even if the command signals are synchronized and therefore can lead to combustion instabilities.